1. Field of the Invention
The present invention relates to an optical head including a plurality of light sources having different wavelengths and adapted to optically record or reproduce information on or from a plurality of types of information recording media such as optical discs, an optical disc device including such an optical head and an information recording and/or reproducing device including such an optical disc device.
2. Description of the Background Art
In recent years, with the practical use of blue-violet semiconductor lasers, Blu-ray discs (hereinafter, “BDs”), which are optical information recording media (hereinafter, also referred to as “optical discs”) having the same size as CDs (Compact Discs) and DVDs (Digital Versatile Discs), high density and large capacity, have been put to practical use. These BDs are optical discs which record or reproduce information using a blue-violet laser light source for emitting blue-violet light having a wavelength of about 400 nm and an objective lens whose numerical aperture (NA) is increased to 0.85 and whose protective substrates are about 0.1 mm in thickness.
Accordingly, there have been proposed compatible optical heads for recording or reproducing information by focusing laser beams of different wavelengths on information recording surfaces of optical discs whose protective substrates differ in thickness using a single or a plurality of objective lenses.
For example, Japanese Unexamined Patent Publication No. 2002-15456 (prior art) discloses an optical head for recording or reproducing information by focusing laser beams of different wavelengths emitted from separate light sources using a single objective lens while causing the optical axes of the laser beams to substantially coincide using a flat beam splitter.
FIG. 22 is a diagram showing the construction of a conventional optical head. In FIG. 22, an optical head 140 is provided with a first light source 101 for emitting a red laser beam, a first flat beam splitter 102, a first auxiliary lens 103, a second flat beam splitter 104, a collimator lens 105, a mirror 107, an objective lens 108, a detection lens 110, a second light source 111 for emitting an infrared laser beam, a second auxiliary lens 112 and a light receiving element 120.
A DVD 70 is an optical disc with a protective substrate having a thickness of 0.6 mm, and a CD 80 is an optical disc with a protective substrate having a thickness of 1.2 mm.
First of all, an operation of the optical head 140 to record or reproduce information on or from the CD 80 is described. An infrared laser beam emitted from the second light source 111 is converted into a divergent beam having a different NA by the second auxiliary lens 112 and reflected by the second flat beam splitter 104. Thereafter, the infrared laser beam is focused as a light spot on an information recording surface of the CD 80 via the protective substrate by the objective lens 108 after being converted into a substantially parallel beam by the collimator lens 105 and reflected by the mirror 107.
The infrared laser beam reflected by the information recording surface of the CD 80 passes through the objective lens 108 again, is reflected by the mirror 107 and converted into a convergent beam by the collimator lens 105. Thereafter, the infrared laser beam passes through the second flat beam splitter 104 and the first flat beam splitter 102, has astigmatism given thereto by the detection lens 110 and is introduced to the light receiving element 120.
Next, an operation of the optical head 140 to record or reproduce information on or from the DVD 70 is described. A red laser beam emitted from the first light source 101 is converted into a divergent beam having a different NA by the first auxiliary lens 103 and reflected by the first flat beam splitter 102. Thereafter, the red laser beam is focused as a light spot on an information recording surface of the DVD 70 via the protecting substrate by the objective lens 108 after passing through the second flat beam splitter 104 and being converted into a substantially parallel beam by the collimator lens 105 and reflected by the mirror 107.
The red laser beam reflected by the information recording surface of the DVD 70 passes through the objective lens 108 again and is reflected by the mirror 107 and converted into a convergent beam by the collimator lens 105. Thereafter, the red laser beam passes through the second flat beam splitter 104 and the first flat beam splitter 102, has astigmatism given thereto by the detection lens 110 and is introduced to the light receiving element 120.
The first flat beam splitter 102 has a spectral characteristic with a transmittance of about 100% and a reflectance of about 0% for infrared laser beams while having a spectral characteristic with a transmittance of about 50% and a reflectance of about 50% for red laser beams.
On the other hand, the second flat beam splitter 104 has a spectral characteristic with a transmittance of about 50% and a reflectance of about 50% for infrared laser beams while having a spectral characteristic with a transmittance of about 100% and a reflectance of about 0% for red laser beams.
Here, an angle of inclination “a” of the second flat beam splitter 104 is set to 35°, an angle of inclination “b” of the first flat beam splitter 102 is set to 45° and the second flat beam splitter 104 is formed thin from a hard material. Thus, astigmatism and coma aberration produced in an optical path up to the optical disc can be suppressed.
The construction of the optical head in which the first and second flat beam splitters 102 and 104 are both formed by parallel plates is disclosed in the above prior art.
However, a spectral characteristic of the flat beam splitter with respect to a polarization direction of an incident laser beam is not described in the prior art at all. In other words, since the flat beam splitters of the prior art are wavelength selecting prisms only specifying the transmittances and reflectances of infrared laser beams and red laser beams, a so-called isolation performance of improving light utilization efficiency by a combination of a wave plate and a polarization beam splitter in the form of a flat plate is not described. Accordingly, problems peculiar to the polarization beam splitter in the form of a flat plate such as incident angle dependency when a laser beam of a specified polarization direction is incident are not mentioned at all. For example, problems such as the one that power necessary for a laser light source increases or a light quantity distribution of a far field pattern of a laser beam incident on an objective lens becomes asymmetric due to insufficient light utilization efficiency, thereby deteriorating recording performance, occur in a recording optical head.
Further, the optical head disclosed in the prior art is an optical head for recording or reproducing information on or from DVDs and CDs. Accordingly, neither disclosure nor indication is made regarding problems peculiar to the optical head for recording or reproducing information on or from high-density optical discs such as BDs using a blue-violet light source such as the construction of the optical head associated with different directions of far field patterns suitable for recording or reproduction between BDs and DVDs and resistance characteristics of lens glass materials against irradiation of blue-violet laser beams. In other words, the construction of the optical head disclosed in the prior art cannot be applied to an optical head for recording or reproducing information on or from high-density optical discs such as BDs.